The proposed study will investigate the role of posttranscriptional regulation in the expression of the oogenetic program of information during development. This program which includes oogenetic (maternal) mRNA and possibly other informational elements is layed down in the egg during oogenesis, and subsequently directs the majority of protein synthesis and determines the initiation of differentiation in early embryogenesis. This study will concentrate on three aspects of this problem: (1) the stimulation of protein synthesis at fertilization with emphasis on the structure and role of masked messenger RNA which exists in the form of messenger ribonucleoprotein particles; (2) translation level control of the recruitment of mRNA from the mRNP pool into functioning polysomes with emphasis on the use of cloned cDNA probes to examine the behavior of specific mRNAs; and (3) the role of stored mRNA in localization phenomena with emphasis on the synthesis and localization of plasma membrane proteins in the 16-cell stage embryo.